Diatom Assemblage in a Tropical Lake of Northeastern Brazil

The composition and spatial variation of diatom assemblage in surface sediments of Caçó Lake (shallow, mesotrophic and weakly acid lake – Maranhão State, Brazil) were analyzed in order to know the distribution pattern of the species along the lake during rainy season (April 1999). Four zones were established in the lake based on 21 diatoms species and habitat affinities. The first three zones (prime three meters deep to six meters deep) were marked by the occurrence of Pinnularia gigas, Frustulia rhomboides, Encyonopsis krasskei, Eunotia camelus, E. femoriformis and E. monodon. Zone IV (seven to nine meters deep) was inhabited mainly by Surirella biseriata and Fragilariforma floridana. During the beginning of the rainy season, the diatom assemblage in Caçó Lake was composed mainly by benthic and epiphytic forms that reflected the low lake levels and the abundance of littoral vegetation present in this lake. Key word: diatoms, shallow lake, surface sediments, Caçó Lake, Brazil Author for correspondence: [email protected] INTRODUCTION Lakes are intricately tied to the climate system, and their water level and chemistry are a manifestation of the balance between inputs (precipitation, stream inflow, surface runoff, groundwater inflow) and outputs (evaporation, stream outflow, ground water recharge) according to Mason et al. (1994). Changes in a hydrological budget can be caused by either climatic change or human activity and have the potential to alter lake level, hydrochemistry, hydrodynamic, and nutrient dissolution the lakes. These, in turn, may affect the physiological responses and species composition the lake’s biota, including the diatoms (Fritz et al., 1999). Diatoms are extremely sensitive to conditions prevailing in different aquatic conditions (Gasse et al., 1983). The use of diatom as indicators of environmental quality in rivers, lakes and streams is important for three basic reasons: their importance in ecosystems, their utility as indicators of environmental state, and their fundamental role in food webs (Jan Stevenson and Pan, 1999). For these reasons, the study of the composition and abundance of the diatoms can give valuable information about the extension and temporal variation of the littoral and pelagic zones Nascimento, L. R. do et al. Braz. Arch. Biol. Technol. v.53 n.1: pp. 241-248, Jan/Feb 2010 242 in the aquatic environments. On the other hand, the diatoms are also used as indicator organisms of long-term ecological perturbations, such as climatic changes, eutrophication and acidification (Battarbee, 1986; Wolin and Duthie, 1999; Stoermer and Smol, 1999). However, uncertainties arise when attempting to reconstruct paleoenvironments based on diatoms records because a diatom assemblage preserved in sediments does not precisely correspond to the modern diatom flora (Jiang et al., 2004). Therefore, studies on diatoms from surface sediments and their relationships with the sedimentary environments of the region are necessary before carrying out paleolake reconstruction using fossil diatom records (Jiang et al., 2001; De Sève, 1999; Juggins, 1992). Few data are available about diatoms on surface sediments in South America, mainly in shallow lakes which are common in Brazil. Caçó is located in northeastern Brazil, and its water level is supported by a seasonal rainy period and ground water input. This study was carried out to examine the composition of the diatom assemblage on the surface sediment of a tropical lake from northeastern Brazil, (which has been subjected to seasonal fluctuations in water level), and to investigate the grade of response of diatoms distribution to the water level changes. Some data about diatoms in surface sediments are available in South America, such as Miretzky et al. (2002) in Chascomús Lake (Argentina) and Sylvestre et al. (2001) in Araruama lagoon (Brazil). MATERIAL AND METHODS Caçó Lake (2° 58’S, 43° 25’W) is located in northeastern Brazil (Maranhão State, Fig. 1), 100 km from the Atlantic coast and about 120 m above sea level. It’s corresponds in a valley that has enclosed in an SW-NE oriented former river valley within a dune field dating back to Pleistocene times (Jacob et al., 2004). The climate is tropical humid with 4 5 months of dry season. Mean annual temperature is 26 28°C and with annual precipitation of about 1500 to 1750 mm, rainy season occurs mainly from to November to May, and it is controlled by the Intertropical Convergence Zone (ITCZ). Maximum lake depth varies between 9-12 m during the dry and wet season. The watershed modern vegetation ranges from “Restinga” (local steppe vegetation) to “Cerrado” (Savanna sandy vegetation) on eolian sand (Ceccantini and Ledru, 2000, Sifeddine et al., 2003). Caçó is shallow, slightly acidic, oligotrophic and a polymitic system. The vertical circulation of the water column is basically controlled by the constant action of trade winds in the region. These features associated to the coastal vegetation result in a marked biological zonation with development of four different habitats or biozones (Fig. 2). Zone 1 (the prime 3 meters) and zone 2 (3-4m deep) corresponds the littoral zone with macrophytes and a great amount of plants semi emerged and submerged. Zone 3 (between 5 and 6 meters deep) is characterized by the presence of exclusive submerged vascular vegetation. Finally, Zone 4 (from 7 to 9 meters deep) is characterized by scarce vegetation. Figure 1 Map of study area showing the location of transects samples. Diatom Assemblage in a Tropical Lake of Northeastern Brazil Braz. Arch. Biol. Technol. v.53 n.1: pp. 241-248, Jan/Feb 2010 243 Figure 2 Biological zonation and sites of the sampling zones on the Caçó Lake sediments. Sampling and Analysis Surface sediments samples were collected by scrapping the uppermost 1cm from nine points in an area of nine meters, with collect on at one meter of distance each one along the bathymetric profile, in April 1999 (Fig. 2). About 2 g were homogenized and cleaned for diatom analysis by boiling in 30% H2O2 and 70% HNO3. Cleaned material was dried and about 0.4 mL was evaporated on the cover slips and mounted onto a glass slide with Naphrax for microscopic observation. Diatom identification were based on Meltzeltlin and Lange-Bertalot (1998), LangeBertalot (1996), Lange-Bertalot and Moser (1994), Krammer (1992), Krammer and Lange-Bertalot (1986-1991), Patrick and Reimer (1975, 1966). Water temperature (◦C) and pH were obtained with Horiba equipment, model U-10. Total phosphorus and Nitrate (NO3 ) were determined by HPLC, according to Valderrama (1981). Surface sediments were not dated directly, but assumed that they represented the recent, because the diatom distribution generally fitted well into modern hydrological characteristics at the samples sites (Jiang et al., 2004). Qualitative and quantitative analysis were performed in chamber of 2 mL, using Ütermohl method (1958) at a magnification of 400X. When possible 300 valves were quantified to estimate the concentration of valves.g (Gasse, 1992). The counting method was applied to the total assemblage, i.e, and dead and alive diatoms. The cluster analyses of the relative abundance of the diatoms along the horizontal profile depth were performed using STATISTICA 6.0.